The present invention relates to a polyester film for laminating the external surface of a metal can end substrate. More particularly the present invention relates to a polyester film for producing a metal can end with excellent design flexibility.
Presently, there is a demand to elevate the design quality of the metal can ends (including can covers and can lids), especially in the beverage industry, by making a print on the can ends. However, it has been impossible to make a print on the finished metal can ends, and it has also been very difficult mechanically to make a print on the coiled metal used as a metal can end substrates. Hitherto, therefore, there have been taken only the technically restricted measures such that the printed base metal of cut plates is used as can end substrates. Thus, industry scale production of the metal can ends having excellent design aspects such as aesthetic designs is not easy. Also, in the case of cut plate printing, the kind of ink usable is limited as it is required to use an ink with high viscosity for preventing defacing of fine prints. Further the printing method is also restricted to offset printing since printing is conducted on the hard metal surface.
For solving the above these problems, it has been proposed to laminate a thermoplastic resin film having a printing ink layer on the metal can end.
For instance, in Japanese Patent Application Laid-Open (KOKAI) No. 4-292942, there is proposed a laminating film for metal plates, comprising a thermoplastic resin film having a printing ink layer and an adhesive layer composed of a curing resin formed on another side of the thermoplastic resin film. In Japanese Patent Application Laid-Open (KOKAI) No. 4-292941, there is proposed a laminating film for metal plates, comprising a thermoplastic resin film having a printing ink layer and an adhesive layer composed of a curing resin formed on another side of the thermoplastic resin film, a transparent or colored film formed on the surface of the adhesive layer, a heat sealing layer formed on the surface of the said transparent or colored film.
These laminating films, however, have the environmental problem because of the necessity to use a solvent in forming the adhesive layer composed of a curing resin such as epoxy resin and polyurethane resin. The laminating films also involve the problem of the increased number of processes because at least the thermoplastic resin film forming process, resin laminating process and printing process are required as the production process, and the collateral problem of high production cost.
As the results of the present inventors' for solving the above-mentioned problems, it has been found that a polyester film which contains a color pigment and/or a color dye, or printed on at least one surface thereof, and has a thickness of 3 to 50 .mu.m is advantageous in use for laminating the external surface of a metal can end substrate. The present invention has been attained on the basis of this finding.